The present invention relates generally to semiconductor designs, and more particularly to a semiconductor structure that isolates devices of various operation voltages.
A system on chip (SOC) may include a plurality of circuit areas that operate at different voltage levels. For example, a liquid crystal display (LCD) driver may include various circuit areas that operate at a low (1.8 or 2.5 V) voltage, medium voltage (3.3 or 5 V), and high (30 or 40 V) voltage. The operating voltage of one device can have effects on its neighboring devices that operate at a different voltage. For example, the threshold voltage VT of a metal-oxide-semiconductor (MOS) transistor is often influenced by the voltage on the body. The body effect may create a voltage difference between the body terminals of two devices that operate at different voltage levels. As the voltage on the body varies, the density of electrons in the inversion layer beneath the gate changes, which, in turn, changes the threshold voltage VT.
The voltage on the body is primarily set on the backside of a semiconductor chip. However, there are also other subtle influences. One example is the effect of lateral currents within the chip. Current can escape from circuit elements that are not thoroughly isolated from the rest of the chip. This current may travel laterally in the lower substrate of the chip. Current traversing a resistance induces a voltage drop. So, while the backside of a chip is likely metallized, the voltage may not be the same at all lateral points within the lower substrate beneath active circuit elements. The result is that, even for circuit elements of the same design technology, there can still be differing body voltages. Within a single circuit type, isolation is needed due to the body effect. Moreover, circuits that are designed to operate at different voltages are often designed to operate at different body voltages. Therefore, circuits with different operating voltages cannot be placed on the same semiconductor chip unless they are properly isolated from the influence of the substrate bias voltage. Also, without isolation, there may be punch through between different devices, as the depletion regions of devices that operate at different voltages expand toward each other. A proper isolation design is imperative for a multi-operation-voltages IC.
A typical, conventional isolation structure may occupy a significant amount of layout area on an IC chip. In a circuit that has regions of various operation voltages, the area used for isolation structures increases significantly. Given the constant demand for a more compact IC design, such conventional isolation structure is apparently undesirable.
Therefore, what is needed in the art of semiconductor designs are more compact structures that isolate devices of various operation voltages.